From brideout at haystack.mit.edu  Tue May 25 12:19:38 2004
From: brideout at haystack.mit.edu (William Rideout)
Date: Wed Jul 21 10:29:13 2004
Subject: [gps-developers] New version of B. Rideout's GPS toolkit
Message-ID: <40B3721A.6070000@haystack.mit.edu>

I've updated the GPS toolkit on my webpage 
(http://www.haystack.mit.edu/address/brideout.html).  The toolkit contains all 
the tools to download and process GPS data, and Matlab scripts to plot it.

The new features are:

1. Topex data is now automatically downloaded and processed if available.

2. make_gps_movie_cart and make_scin_movie_cart display Topex data.

3. The following are new matlab scripts:
	- make_gps_diff_cart:  allows you to plot differential TEC with any time 
difference and any integration period.
	- make_gps_diff_cart: same as above in polar coordinates
	- make_scin_movie_cart: overlays scintillation data on gps data.

These new matlab methods are also available on hyperion using Matlab6.  Type 
help <script name> for usage in Matlab.

When plotting the Topex data, you will need to enter a number 
"topexScaleFactor".  Officially, this is the ratio of TEC below 1200 km (as seen 
by Topex) and the total TEC seen by GPS.  However, I think this number should 
always be 1.0, because I think JPL is already scaling the TEC when it creates 
this file.  This is based on two observation:

	a.  Using 1.0 gives a good match when Topex and GPS data overlap near the coast.
	b.  In the JPL Topex file, GIM and IRI estimates of TEC are also given.

However, this is something Anthea still needs to check on, so I'm leaving the 
parameter in for now.

Let me know if you have any questions

Bill

-- 
Bill Rideout
MIT Haystack Observatory
Email: brideout@haystack.mit.edu
Phone: 781 981-5624

From brideout at haystack.mit.edu  Fri May 28 16:27:12 2004
From: brideout at haystack.mit.edu (William Rideout)
Date: Wed Jul 21 10:29:13 2004
Subject: [gps-developers] Downloading Jason TEC data
Message-ID: <40B7A0A0.6070603@haystack.mit.edu>

Anthea and I found the Jason data on the JPL Physical Oceanography Distributed 
Active Archive Center (http://podaac.jpl.nasa.gov/).  It's important to note 
that Jason and Topex fly in tandem only about 15 degrees apart, so the Jason 
data will often overlap the Topex data.  However, there may well be less missing 
Jason data than Topex.

The Jason data is in binary format, but they supply C routines to make reading 
it straightforward.  The data is organized not by day, but by pass, where two 
passes (north-going, then south-going) are an orbit.  They control the satellite 
so that it makes 127 orbits or 254 passes every 9.9156 days, at which time it 
returns to the same point in space.  Each 9.9156 days is called a cycle, 
starting with cycle 1.  Cycle 1 began at 2002-01-15 06:07:06 UTC.

The data is located at:

ftp://podaac.jpl.nasa.gov/pub/sea_surface_height/jason/gdr/data/

in the subdirectory: c<3 digit cycle num>

which contains up to 254 files of the form:

JA1_GDR_2PaP<3 digit cycle num>_<3 digit pass_num>.NASA

These supplied orbital parameters are not precise, because when I tried to 
estimate when cycle 51 should start (about a year and a half later), I was off 
by 50 minutes.  For this reason, when downloading a day of data, I'll use the 
calculation above, but I'll also download the files from the day before and the 
day after.  This should work well beyond the expected lifetime of Jason.

The second part of getting the Jason data will be writing a small C program to 
produce a TEC file in the same form as GPS and topex.  The file contains the 
parameter iono_corr_alt_ku. To calculate TEC, use

TEC = -(iono_corr_alt_ku*f^2)/40.3

where f = frequency in Hz (for the Ku band this is 13.575 GHz).

I guess I'll have Matlab draw colored "J"'s on the map once we have the data.

Bill


-- 
Bill Rideout
MIT Haystack Observatory
Email: brideout@haystack.mit.edu
Phone: 781 981-5624